In the production of hydrocarbons from a well bore through a string of tubing, there are instances where the operator would like to monitor the pressure of the fluids over a period of time as a function of real time. In present systems to obtain a real time pressure measurement, a pressure gauge is attached to the exterior of the string of tubing and the gauge, the tubing and an attached electrical conductor wire are located in a well bore. Should a problem arise with the tool or for any other reason which might require removal of the tool, the well must be killed and the gauge retrieved with the string of tubing. Obviously, this is expensive and time consuming.
It is desirable to have a pressure gauge system-which can be utilized downhole and which can be retrieved for repair or replacement without killing the well and where real time measurements can be obtained.
Side pocket mandrels are commonly used devices in well bore operations, principally for gas lift operations. Side pocket mandrels are specially constructed with an elongated offset chamber to one side of a full opening bore through the mandrel. The offset chamber typically has an elongated pocket which is open at both ends and which is sized to receive a well instrument or tool. The well instrument can be installed in a number of ways in such a side pocket mandrel, including standard or oriented kick over tools, whip stocks or the like. The well instrument is typically installed and removed by a wireline operation.
Side pocket mandrels, as utilized in high temperature and corrosive wells, are constructed from 4130 or similar case hardened steel. One of the problems associated with modification of such mandrels is that any welding or the like requires heat treatment and any appurtenance attached to the mandrel will be subjected to heat treatment. This can produce adverse consequences on any such appurtenances. Another problem of modifying the side pocket mandrel is the existence of internal high pressure in the string of tubing which makes it necessary to prevent intrusion of fluids under pressure to the annulus of the well bore and access of the tubing fluids in the tubing string to the well bore annulus.
In other proposed systems, such as described in the OTC paper 5920, 1989 entitled "A Downhole Electrical Wet Connection System For Delivery and Retrieval of Monitoring Instruments by Wireline", a side pocket mandrel and pressure gauge utilize a downhole "wet connector" for coupling power to a tool and for read out of data. "Wet connectors" in a high pressure, corrosive environment ultimately corrode. In making up the connection, it is often difficult to make connections because of mud or debris in the well bore. Moreover, brine in the fluid causes electrical shorting of circuits. In short, an electrical wet connector is not reliable and this is particularly true over a period of time.
In another type of system known as a "Data Latch" system, a battery powered pressure gauge is installed in a mandrel which has a bypass. A wireline tool with an inductive coil is latched in the bore of the mandrel while permitting a fluid bypass. The inductive coil on the wireline tool couples to a magnetic coil in the mandrel for obtaining a read out of real time measurements. The system does not provide downhole power to the tool and battery failure requires killing the well and retrieving the tool with the well string.
Inductive coupling devices are difficult to construct for a downhole environment and yet are extremely desirable devices for downhole tools as a replacement for the above systems.